We don't eat that late either as our kids are in bed by 7:30pm as well. It's not good to eat too late but, there are families that have no choice- you do and don't forget that families will do what they need to for what is best for their family and you need too as well. You can offer them options which is being more than fair.
I use break to get everything ready for dinner and will put the meat in the oven and turn on the stove around 4:30 and then eat when the daycare kids are gone ~ if I'm not cooking in my crock pot which I LIVE in for convenience.
You could offer them a few options. Let them know that you can have their son join you for dinner with a light snack/drink and or as Reggio suggested, pay extra for him to join you for dinner?